This theory doesn't hold when you pass the center of gravity. Then again, air will become less dense as should be obvious with a basic understanding of physics

The key to understanding Hollow Earth theory is understanding what would be different from a physics perspective if the center of gravitational pull was NOT the center of the earth but rather an electromagnetic coil-like strip about 400 miles underneath the earth's surface.

A basic understanding of physics? Oh! YOU'RE style of physics, you mean. Oh, sure if there were giant coils just under the surface. Yeah, I see.

I think bringing physics into this is just plain ridiculous. And how would the hollow earth have survived the impact from giant meteorites? Such as the one which killed the dinosaurs? And then the problem of earthquakes. I suppose they are just the Molochs dropping things. Yes, I see now.

There are far too many obvious reasons why the earth isn't hollow. But there's no reason why there couldn't be entire worlds down there a few miles deep.